This invention relates to a semiconductor light-emitting device and, more particularly, to a semiconductor light-emitting device having a ridge waveguide type stripe structure, which is suitable for a semiconductor laser device, and a manufacturing method for this semiconductor light-emitting device.
A structure so-called as a ridge waveguide type is frequently used to produce semiconductor light-emitting devices without difficulties. FIG. 2 shows a manufacturing method for such a structure. First, a first conductivity type cladding layer 202, an active layer 203, a second conductivity type cladding layer 204, and a second conductivity type contact layer 205 are grown on a substrate 201. A photoresist 211 having stripe openings as a pattern made by photolithography is formed on a wafer surface to form a stripe-shaped ridge 209 by a wet etching process using the photoresist 211 as a mask so that the second conductivity type cladding layer remains with a prescribed thickness. An insulating protective film 206 such as SiNx is subsequently formed on the whole surface on an epitaxial side, and only the protective film at the top of the ridge is removed by etching using a photoresist having stripe openings as a pattern made by photolithography. This structure prevents a current from flowing through portions other than the top of the ridge. Another layer of protective film may further be formed on the ridge side surface. Then, an epitaxial side electrode 207 and a substrate side electrode 208 are formed.
According to this structure, currents are injected into the active layer 203 after injected through the ridge portion 209 of the cladding layer. Currents are thus concentrated into the active layer region under the ridge portion 209, thereby generating light having a wavelength corresponding to the band gap of the active layer. At that time, the band gap of the active layer is ordinarily smaller than those of the upper and lower cladding layers, and the refractive index of the active layer is larger than those of the upper and lower cladding layers, so that carriers and light can be confined effectively in the active layer. Because the protective film 206 having a smaller refractive index than the semiconductor portions is formed at a non-ridge portion 210, the effective refractive index of the active layer region under the non-ridge portion 210 becomes smaller than that of the ridge portion 209. Consequently, the generated light is confined in the active layer region under the ridge portion 209. This structure thus can stabilize the transverse mode for laser oscillation and can reduce the threshold currents.
With such a conventional manufacturing method for ridge waveguide type semiconductor light-emitting device, the ridge portion is formed by the etching process, so that it is difficult to accurately control the thickness of the cladding layer at the non-ridge portion 210. As a result, the effective refractive index at that portion largely varies due to slight differences of the thickness of the cladding layer at the non-ridge portion, thereby deviating the laser characteristics of the semiconductor light-emitting device, and rendering product yields hardly improve. Where a laser device of a single transverse mode is produced, a very highly accurate alignment technique is required, because it is difficult to use process simplifying techniques such as a self-alignment in the conventional manufacturing method, though the top width of the ridge portion is at most about several microns. Such a complicated, fine photolithographic technology makes device production steps complicated and device production yields reduced. If a SiNx film is formed on the ridge side wall, a deletion layer of about 0.1 micron may be formed on the surface side of the ridge side wall to narrow the effective current channel width, thereby raising a problem that the pass resistance becomes larger.
Meanwhile, as a light source for information processing to improve the recording density, visible laser devices (ordinarily 630  to 690 nm) using AlGaInP basis in lieu of conventional AlGaAs basis (wavelength about 780 nm) are put to practical use, but the following researches have been made to realize shorter wavelength, lower threshold, and high temperature operation.
In a production of an AlGaInP/GaInP based visible laser device, use of a substrate having an off-angle from the (100) plane toward the [011] direction (or [0-1-1] direction) allows the band gap from narrowing due to formation (ordering) of natural super lattices, thereby rendering the wavelength shorter readily, facilitating high concentration doping of p-type dopants (e.g., Zn, Be, and Mg), and improving the oscillation threshold current of the device by enhancement of the hetero-barrier and temperature characteristics. If the off-angle is too small, step bunching appears outstandingly, and large undulations are formed at the hetero-boundaries, so that a shift amount in which the PL wavelength (or oscillation wavelength) is shortened by quantum effects to the bulk active layer may be smaller than the designed amount where a quantum well structure (GaInP well layer of about 10 nm or less) is manufactured. If the off-angle is made larger, the step bunching is reduced, and the hetero-boundaries become flat, thereby making the wavelength shorter by the quantum effect as designed. Thus, a substrate having an off-angle of 6 to 16 degrees from the (100) plane toward the [011] direction (or [0-1-1] direction) is generally used to suppress formation of natural super lattices and generation of step bunching, which impede the wavelength from becoming shorter, as well as to suppress the oscillation threshold current from increasing due to shortened wavelength from p-type high concentration doping and impairment of temperature characteristics. A proper off-angle should be selected in consideration of thickness of the GaInP well layer and the stress amount depending on the targeted wavelength such as 650 nm or 635 nm.
When natural super lattice is formed in the active layer, it is deformed to be mixed crystal during current injection whereby problems may be raised such that oscillation wavelength or emission wavelength is changed and device properties are impaired. Natural super lattice is easily formed in an active layer made of materials including In and Ga as constituent elements such as GaInAs, AlGaInAs, InGaAsP as well as the above-mentioned GaInP and AlGaInP. Use of an off-angle substrate suppresses formation of natural super lattice and effectively solves the problems.
To reduce waveguide loss and mirror loss, a resonator is formed in extending in a striped shape as much as vertical to the off-angled direction of the substrate. FIGS. 3(a) and 3(b) show cross sections of conventional ridge and groove type inner stripe structures made of a semiconductor using a current block layer. In FIGS. 3(a), 3(b), numeral 301 is a substrate; numeral 302 is a first conductivity type cladding layer; numeral 303 is an active layer; numeral 304 is a second conductivity type cladding layer; numeral 305 is a first conductivity type current block layer; numeral 306 is a second conductivity type contact layer; numeral 307 is an epitaxial side electrode; numeral 308 is a substrate side electrode; numeral 311 is a substrate; numeral 312 is a first conductivity type cladding layer; numeral 313 is an active layer; numeral 314 is a second conductivity type first cladding layer; numeral 315 is a first conductivity type current block layer; numeral 316 is a second conductivity type second cladding layer; numeral 317 is a second conductivity type contact layer; numeral 318 is an epitaxial side electrode; and numeral 319 is a substrate side electrode. In this situation, because the shape of the ridge or groove may become horizontally asymmetric or the optical density profile may become horizontally asymmetric due to the off-angle of the substrate, problems may be raised such that a stable fundamental transverse mode required for a laser diode for information processing such as for optical discs may not be easily obtained, that kink level may be lowered, and that the horizontal asymmetry of the beam profile may increase. Particularly, in the case of real refractive index guide in which ends of the optical profile come out to the block layer, this problem may become apparent.
In consideration of those problems in the conventional art, it is an object of the invention to provide a semiconductor light-emitting device having a ridge waveguide type stripe structure which can be manufactured in a simple way with stable laser property. It is also another object of the invention to provide a semiconductor light-emitting device having a stable fundamental transverse mode at a high power operation stage where the horizontal symmetry of the ridge shape of the ridge waveguide type laser is almost not affected by the horizontal asymmetry of the optical intensity profile even where a substrate having a large off-angle for shortening the wavelength as for the AlGaInP/GaInP based visible laser diode is used. It is yet another object of the invention to provide a method for manufacturing semiconductor light-emitting device with good production yield in a simplified step for producing such a device without requiring any complicated, fine photolithographic technology.
The inventors have discovered, upon extensive researches to accomplish the above objects, that covering both sides of a stripe region with a protective film makes a complicated, fine photolithographic technology unnecessary, simplifies a manufacturing process for the device, and greatly improves production yield of the device. The inventors also found that the semiconductor light-emitting device having such a structure can be manufactured easily by a selective growth using the protective film, and reached the invention upon finding that a semiconductor light-emitting device can obtain a stable fundamental transverse mode at a high power operation stage where the horizontal symmetry of the ridge shape of the ridge waveguide type laser is almost not affected by the horizontal asymmetry of the optical intensity profile even where a substrate having a large off-angle for shortening the wavelength as for the AlGaIn/GaInP based visible laser diode is used.
That is, this invention is to provide a semiconductor light-emitting device comprising a substrate having a surface having an off-angle to a crystallographic plane of low-degree surface orientation, the substrate having thereon: compound semiconductor layers including an active layer; a selective growth protective film formed on the compound semiconductor layers and having an opening at the region corresponding to a stripe region to which a current is injected; and a ridge-shaped compound semiconductor layer formed to cover the opening.
In another aspect of the invention, a semiconductor light-emitting device comprises a substrate having a surface having an off-angle to a crystallographic plane of low-degree surface orientation, the substrate having thereon: compound semiconductor layers including an active layer; a protective film formed on the compound semiconductor layers and having an opening at the region corresponding to a stripe region to which a current is injected; and a ridge-shaped compound semiconductor layer formed to cover the opening, wherein at least a portion of a side wall of the ridge-shaped compound semiconductor layer has a forward mesa shape.
In the semiconductor light-emitting device according to the invention, preferably, the compound semiconductor layers includes an active layer further include a first conductivity type cladding layer and a second conductivity type first cladding layer, and ridge-shaped compound semiconductor layer includes a second conductivity type second cladding layer. No protective film is preferably formed either on a top portion or side surfaces of the ridge-shaped compound semiconductor layer, and a contact layer may be formed as to cover the entire surface of the top portion and the side surfaces of the ridge-shaped compound semiconductor layer. The ridge-shaped compound semiconductor layer is preferably formed as to cover a portion of the surface of the protective film. The crystallographic plane of the low-degree surface orientation of the substrate may be a (100) plane or a plane crystallographically equivalent to a (100) plane; the off-angle may be 30 degrees or less; and a direction of the off-angle is preferably within xc2x130xc2x0 from a direction perpendicular to a longitudinal direction of the stripe region. A longitudinal direction of the stripe region may be within xc2x130xc2x0 from a [0-11] direction or a direction crystallographically equivalent to a [0-11] direction, and the off-angle direction may be preferably within xc2x130xc2x0 from a [0-11] direction or a direction crystallographically equivalent to a [0-11] direction. The active layer is preferably, an AlGaInP layer or a GaInP layer, and the substrate is preferably made of a zinc-blende type crystal such as GaAs. An oxidation suppressing layer may be preferably provided between the protective film and the compound semiconductor layers including an active layer so that the oxidation suppressive layer covers the semiconductor layers including an active layer at the opening of the protective layer.
In yet another aspect of the invention, a method of manufacturing semiconductor light-emitting device comprises the steps of: growing a compound semiconductor epitaxial layer including an active layer on a substrate having a surface having an off-angle to a crystallographic plane of low-degree surface orientation; forming a protective film having an opening on a surface of the compound semiconductor epitaxial layer; and selectively growing a ridge-shaped compound semiconductor epitaxial layer to cover the opening.
In the method of manufacturing semiconductor light-emitting device according to the invention, preferably, the compound semiconductor layers includes an active layer further including a first conductivity type cladding layer and a second conductivity type first cladding layer, and ridge-shaped compound semiconductor layer includes a second conductivity type second cladding layer. The second conductivity type second cladding layer is preferably grown as to cover a portion of a surface of the protective film.